A conventional process for preparing a polyester shrinkable film is batchwise and, as shown in FIG. 1, includes steps of: (i) subjecting a stock mixture containing a diacid component and a diol component to esterification at an elevated temperature in an esterification reactor 1 to obtain a reaction mixture; (ii) subjecting the reaction mixture to polymerization at an elevated temperature in a polymerization reactor 2 to obtain a polyester melt; (iii) subjecting the polyester melt to cooling and then pelletizing in a pelletizer 3 to obtain polyester pellets, and storing the polyester pellets in a storage tank 4; (iv) subjecting the polyester pellets to drying and introducing the dried polyester pellets into an extruder 5 to forma polyester melt by heating; and (v) introducing the polyester melt into a T-die 6, a casting device 7, and a stretching device 8 in sequence to prepare the polyester shrinkable film.
In step (iv) of the batchwise process, the polyester pellets should be reheated at a temperature usually from 230° C. to 250° C. in the extruder 5 to form the polyester melt. Such a step may result in thermal cracking of the polyester melt, leading to a reduction in the intrinsic viscosity of the polyester melt, which may affect mechanical properties of the polyester shrinkable film. In order not to affect the mechanical properties of the polyester shrinkable film, the intrinsic viscosity of the polyester melt should be raised to a value, for example, above 0.7 in the aforesaid step (ii). However, in order to obtain a higher intrinsic viscosity for the polyester melt, it is necessary to raise a reaction temperature and/or extend a reaction period in step (ii), which may result in deterioration of hue of the polyester shrinkable film thus prepared. In addition to the aforesaid shortcoming, the batchwise process involves the processing steps of cooling, pelletizing, transporting, storing, drying, etc. Some of these processing steps are performed in an open environment, which may cause a problem of contamination by extraneous substances. Furthermore, it is required that the polyester pellets be reheated for hot melt extrusion through the extruder 5 prior to a subsequent film-forming treatment. In addition, there is a difference in residence time between an initial pelletization stage and a subsequent pelletization stage, leading to a variation in viscosity of the polyester, which may affect the stability of the subsequent film-forming treatment. Moreover, since the batchwise process involves the aforesaid various processing steps, it is costly.
In view of the various problems mentioned above, it is desirable to modify the batchwise process to a continuous process. In this case, it is primarily desirable to eliminate steps (iii) and (iv) of the batchwise process so as to avoid the shortcoming associated with the reheating of the polyester pellets. In other words, it is desirable to directly introduce the polyester melt in step (ii) into the T-die 6. However, since the polyester melt in step (ii) usually has a relatively low melt viscosity, it cannot be directly introduced into the T-die 6 without raising the melt viscosity thereof.
CN 103059523 discloses an in-situ polymerization tape-casting method for preparing a biaxial stretching polyester film. The polyester used in the method is of high crystallinity so as to maintain a high melt viscosity sufficient for the subsequent film-forming treatment at an elevated temperature (for example, 265° C.). However, since the polyester is of high crystallinity, the polyester may crystallize when the temperature of the T-die is lower than a melting point of the polyester. The polyester crystals thus formed may adhere to the T-die such that the polyester prepared accordingly has increased flow marks.
Low-crystalline or amorphous polyester may not be used in the method disclosed in CN 103059523 due to insufficient crystallinity thereof. In the preparation of a stretchable film of low-crystalline or amorphous polyester, the melt viscosity of the polyester is usually increased by raising the reaction temperature and/or extending the reaction period in the polymerization step so as to provide the polyester with a sufficiently high melt viscosity in the T-die. However, as described above, raising the reaction temperature and/or extending the reaction period may result in deterioration of hue of the shrinkable film thus prepared.